Such optical pointing devices using laser illumination device are already known in the art. US patent application No. 2006/0226346 A1, filed in the name of the same Assignee, for instance discloses, as shown on FIG. 2, an optical pointing device including a photodetector array 100 including a plurality of pixels, this photodetector array 100 being coupled to processing means 110 (or motion detection processing circuit) for processing the signals outputted by the photodetector array 100. A comparator array 120 may be interposed between processing means 110 and array 100, this comparator array 120 including a plurality of comparator circuits each for comparing the light intensity of a first pixel of array 100 with the light intensity of a second pixel of array 100 and for outputting resulting motion feature conditions.
The optical pointing device further comprises at least one coherent light source 130 such as a laser illumination source, which produces radiation at a determined flash rate, that impinges with a determined gradient on a portion of a surface S. Surface S may be a planar or non-planar surface, such as a surface over which the pointing device is moved (as in the case of an optical mouse), the surface of a ball (as in the case of an optical trackball) or any other suitable surface that may provide an appropriate speckled intensity pattern for detection by photodetector array 100.
Processing means 110 is further adapted to communicate in a bi-directional manner with an interface 140 that communicates in turn with a host system (not illustrated) over a bus 150. Cursor control signals (and eventually other signals related to the optical pointing device) are supplied to the host system over bus 150. Processing means 110 may also receive information, such as configuration signals, over bus 150 from the host system. Processing means 110 is essentially designed to intermittently sample the pixel outputs of photodetector array 100 in accordance with a defined sequence. The information of two successive samples or speckled images is compared and a relative motion measurement is extracted by processing means 110. The adequate cursor control signals are then derived from the relative motion measurement and transmitted to the host system via line interface 140.
Nevertheless, laser driven optical pointing device such as mice are required to meet stringent optical safety requirements. Actually, since an optical pointing device may be handled easily by the user, the laser could be potentially directed towards someone's eye. This necessitates the need for a fault tolerant design such that a single point failure anywhere in the laser driver path will not result in a harmful laser output power.
These safety requirements eventually along with additional system requirements create a difficult control problem in which the optical safety and system operation must exist simultaneously. A simple algorithm would consist in setting the laser power low enough that there is no danger of violating the laser power limits. However, this approach is not optimum because there is not sufficient light recovered by the photodetector to support high tracking speeds and/or high signal to noise ratio.